Properties of fats and oils can be modified by an interesterification reaction of the fats and oils with a fatty acid or lower alcohol ester thereof (hereinafter referred to as "fatty reactant") in the presence of an enzyme to introduce a specific fatty acid residue into triglycerides to increase a concentration of specific triglyceride molecular species. However, this reaction is an equilibrium reaction and, therefore, when a target concentration of the desired triglyceride molecular species is higher, a larger amount of an expensive feedstock fatty reactant is required. Then, after the reaction, the fats and oils are subjected to fractionation such as solvent fractionation to obtain the desired target concentration with a limited amount of a feedstock fatty reactant.
However, solvent fractionation requires large facilities and therefore high costs. In addition, from a safety viewpoint, to use a solvent should be minimized. Then, if a concentration of the desired triglyceride molecular species could be sufficiently increased only by an interesterification reaction with a limited amount of a feedstock fatty reactant but without any solvent fractionation, it would be desirable.
As for a process for improving a reaction efficiency of a fatty reactant, a multistage interesterification reaction has been known. However, any sufficient efficiency cannot be obtained by merely increasing the number of the stages. And, isomerization is caused by migration of triglycerides and diglycerides during a distillation step of one stage, which results in formation of undesirable by-products during the reaction in a subsequent stage. For example, in case of production of SOS-type hard butter from an oleic fat or oil and a stearic acid ester (S: stearic acid as a constituent fatty acid of a triglyceride, O: oleic acid as a constituent fatty acid of a triglyceride), an isomer SSO is formed and, in the reaction of the next stage, SSS is formed. Then, it is necessary to fractionate such by-products. In addition, problems such as deterioration of color due to heat history and lowering of stability to deterioration by oxidation are caused.
On the other hand, JP-A 5-219971 discloses a process for multistage interesterification of fats and oils, wherein a fatty reactant used in an interesterification reaction of one stage as such is fed to the reaction of the next stage to improve the reaction efficiency of the fatty reactant. However, such problems as by-products, deterioration of color and lowering of stability to deterioration by oxidation are not yet solved.
JP-B 3-69516 discloses a process for interesterification of fats and oils, wherein a fatty acid ester is separated from a reaction mixture and, after hydrogenation, it is returned to another reaction mixture.
However, to leave fats and oils together with a specific fatty reactant in a reaction mixture has not been known in the prior art.